The present invention relates to an imaging lens, and more particularly to an optical imaging lens for mobile phones or image sensors such as CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) formed by three elements of lens with features of short back focal length, short overall (total) length and low cost.
Due to fast development of modern technology, electronics have becoming more compact and multifunctional. A lot of electronics such as digital cameras, PC (personal computer) cameras, network cameras, mobile phones, even personal digital assistants are equipped with image capturing devices. For convenience of easy-carrying and requirement of humanization, the image capturing devices requires good image quality, compact volume and low cost. Especially for applications on mobile phones, the above requirements are getting more important.
Due to many materials available for making the lens and better correcting effect for color aberration, conventional spherical ground glass lenses have been widely adopted in optical industry field. However, the spherical ground glass lenses have poor effect on correction of spherical aberration and astigmatic aberration when being applied under condition of a smaller f number and wide angle of field. In order to improve above disadvantages of the conventional spherical ground glass lenses, the image capturing devices available now use aspherical plastic lenses or aspherical molded glass lenses to replace conventional spherical ground glass lenses for better imaging quality. For example, refer to the lenses disclosed the optical imaging lens are formed by three lens elements, such as US2007/0091457, U.S. Pat. Nos. 6,515,809 , 7,262,925, US 2007/0195432, US2005/0128334, JP 2007-121820, JP2005-352317, JP 2004-163786, JP 2007-094113, JP 2005-338234, JP 2007-047513, and JP 2006-098976 etc. The differences among embodiments of above patents are in components or combinations of components. For example, the three lens elements are all in meniscus shape or the first and the second lens elements are meniscus while the third lens element is in plano-concave shape or plano-convex shape. Or the convex surface/concave surface of the first/second/third lens elements can be arranged on the object/image side. Or the refractive power of the three lens elements may vary. For example, refractive power of the first, the second and the third lens elements are respectively positive, negative, positive or positive, positive, negative in sequence. Moreover, the three lens set/lens element respectively satisfy various optical parameters such as f (effective focal length of the whole optical imaging lens), d (distance between the object side and the image side of the first lens element), R11 (radius of curvature of the object side of the first lens element), R12 (radius of curvature of the image side of the first lens element), f1 (effective focal length of the first lens element), f2 (effective focal length of the second lens element), and f3 (effective focal length of the third lens element), as the embodiment shown in Japanese patent No. 3717488. Thus it is learned that the design of a three-element type optical imaging lens depends on optical requirements that result from combinations of various lens in different shapes and with different functions.
In recent years, the optical imaging lens are getting more compact with shorter focal length and better aberration correction for being applied to various portable electronics such as cameras, camera phones, and PDA. In different designs of compact three-element type optical imaging lens, the lens having a first lens element with positive refractive power, a second lens element with negative refractive power, a third lens element with an inflection point that changes the refractive power (also called M-shaped lens) achieves most compact requirement. As to different imaging method, the lens having a first lens element with positive refractive power, a second lens element with negative refractive power, a third M-shaped lens element with positive refractive power are disclosed in EP1830210, JP2008-139853, JP2006-178328, U.S. Pat. Nos. 7,397,613, 7,486,328, 7,423,817, 7,468,847, 7,515,358, US2007/0195426, US2007/0217034, US2007/0229986, US2008/0239510, TWM343167, TW200639432, CN1670560, CN1873460, and so on. The lens having a first lens element with positive refractive power, a second lens element with negative refractive power, a third M-shaped lens element with negative refractive power are disclosed in EP1840618, EP1942363, U.S. Pat. Nos. 7,460,315, 7,460,314, 7,450,323, 7,511,899, US2007/0229987, US2008/0225401, US2008/0266679, US2007/0195426, JP3816093, JP2008-276200, JP2008-233222, JP2007-010773, WO2007039980, CN1945372, and so on.
Applying for optical imaging lens, especially an optical imaging lens for mobile phones and network camera, the users are in urgent need of the imaging lens system with small effective radius, short overall length, short back focal length, and good aberration correction. However, in order to achieve the above-mentioned needs, the prior arts utilize different composition of lenses, different shapes of lenses or different related optical parameters. In order to achieve the convenience for designing and manufacturing the lens, this present invention provides an optical imaging lens with short back focal length, especially utilize a second lens with a relative longer focal length so as to decrease the back focal length effectively for being applied to mobile phones or imaging system more easily.